1. Field of the Invention
This invention is concerned with a means for adjusting the sensitivity of a ceramic-crystal seismic detector.
2. Discussion of the Prior Art
This invention represents an improvement in the method of manufacture and tuning of twin-crystal seismic detectors such as hydrophones or accelerometers for example.
In its simplest form, a detector consists of a hollow housing, usually cylindrical, of suitable material. The top and bottom of the housing are closed by thin metal diaphragms, each supporting a disc-shaped piezo-electric ceramic crystal element cemented to one side thereof. Suitable conductors, electrically connected to the crystal elements, provide means for transmitting the signals from the crystal to the outside world.
In the manufacture of the detector, it is customary to secure the metal diaphragms in place by simply crimping the ends of the housing over the diaphragms. Alternatively the diaphragms were cemented in place or ultrasonically welded.
It is important that the sensitivities of the two crystal elements be matched, preferably within one percent or less. For that reason, it was necessary to hand-pick each pair of crystal elements before assembling the detector. Hand selection necessarily increased production costs.
An additional problem arose in that, once the crystal elements were permanently fixed in place during assembly, a cracked crystal could not later be replaced. The detector was therefore discarded.
In another application, using single-element detectors, a related problem arises when a plurality of such detetors are electrically coupled together as units of an array. In such an array, it is preferable that all of the units have matched sensitivities. Here again, it was necessary to hand-pick the detectors destined to form the units of a proposed array.